Question

In later years, motors improved and could be run directly from a 60
Hz power supply.
As a result, 25 Hz power systems shrank and disappeared. However,
there were
many perfectly good working 25 Hz motors in factories around the
country that
owners were not ready to discard. To keep them running, some users
created their
own 25 Hz power in the plant using motor-generator sets. A
motor-generator set
consists of two machines connected on a common shaft, one acting as
a motor and
the other acting as a generator. if the two machines have different
numbers of poles. but exactly the same shaft speed, then the
electrical frequency of the two machines will be different due to
Equation (3- 34). What combination of poles on the two machines
could convert 60 Hz power to 25 Hz power?